Magnetic field sensors are widely used for isolated current measurement, electronic compasses, position sensors, etc. While some applications, such as current sensing, still primarily operate in the analog domain, other applications such as navigation aids require digital output for easy integration into more complex systems. The present design is particularly applicable, but not limited to, digital output magnetic sensor systems. Possible applications are industrial navigation systems, mobile devices including position sensing and navigation systems, non-invasive sensing of weak magnetic fields and different types of consumer applications.
Examples of magnetic field sensors or magnetic field sensors are Hall-effect magnetometers using Hall plates, fluxgate magnetometers, and magneto-resistive sensors, without being limited to this type of sensors. Magnetic sensors or magnetometers provide an electric signal which is a function of an external magnetic field applied to the sensor. Recent progress in fabrication technologies has enabled the development of miniaturized fluxgate sensors, called micro fluxgate sensors which allow the development of new applications, such as portable electronic compasses, which require low-cost, low power, low noise integrated accurate magnetic field sensors. Digital interfaces can be well adapted to fluxgate magnetometers and miniaturized fluxgate magnetometers, considering their simple implementation in regard to high performance and adjustability.
The following description will focus on fluxgate magnetic sensors but the approach is equally applicable to Hall-based magnetic sensors as well as other magnetic field sensors or magnetometers.
There is a number of demands which a digital magnetic sensor solution should meet, in particular when the solution is to be provided for a mobile application. Primary specifications are low-power dissipation, small size and low costs, but also high sensitivity and high resolution. The architecture described has been developed to improve on currently-available sensors and associated signal acquisition electronics in one or more of these aspects.
Hall-effect magnetometers produce a voltage proportional to the applied magnetic field and also are able to sense polarity. They easily lend themselves to digitization but due to their low sensitivity are useful mainly in applications where the magnetic field strength is relatively large.
Fluxgate magnetometers provide a higher sensitivity and hence are useful also in applications where the magnetic field is weak and changing slowly. A fluxgate magnetometer consists of a small, magnetically susceptible core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation; i.e., saturated, unsaturated, inversely saturated, unsaturated, saturated, and so forth. This constantly changing field induces an electrical voltage or EMF (electro-magnetic force) in the second coil, and this output signal is measured by a detector. In a magnetically neutral background, the EMF generated in the two core halves will have the same magnitude but will be opposite in direction so that it will add up to zero. However, when the core is exposed to a background field, it will be more easily saturated in alignment with that field and less easily saturated in opposition to it. Hence the EMF induced in the two core halves will be out of step. The extent to which this is the case will depend on the strength of the background magnetic field and will determine the output signal of the sensor.
Fluxgate magnetometers offer low noise and high sensitivity but the output signal is depending on the magnetic properties of the core material and it is inherently non-linear. To improve measurement precision, the sensors are often operated in a closed-loop configuration. In this operation mode, a magnetic field is generated via a compensation current flowing through the loops of a sensor coil or an independent compensation coil to oppose the external magnetic field that the sensor sees. The compensation current is generated based on the sensor output and the amount of compensation current is regulated by feedback to be proportional to the external field. By feeding back the compensation current, the response of the sensor can be made linear. Different solutions have been proposed as signal acquisition electronics for fluxgate magnetometers: R. Gottfried-Gottfried et al. describe in “A Miniaturized Magnetic-Field Sensor System consisting of a Planar Fluxgate Sensor and a CMOS Readout Circuit”, Sensors and Actuators, Volume 54, June 1996, pages 443-447, a signal acquisition electronics to be used with a fluxgate sensor wherein the fluxgate output is amplified, demodulated and filtered and used to generate a compensation current. A separate analog-to-digital converter generates the digital output signal of the electronics. This solution requires high precision analog blocks and hence is larger and requires high power. It also is inherently slow due to the amount of filtering required in the feedback. S. Kawahito et al. describe in “A Digital Fluxgate Magnetic Sensor Interface using Sigma-Delta Modulation for Week Magnetic Field Measurement”, IEEE Instrumentation and Measurement Technology Conference, Anchorage, USA, 21.-23. May 2002, pages 257 to 260, micro fluxgate sensors using a delta-sigma converter with a one-bit feedback DAC for generating a compensation current which is applied to a feedback coil. The feedback coil generates a magnetic field which compensates the measured external field. Also this approach requires additional filtering and is inherently slow. F. Gayral et al. describe in “A Sigma-Delta Closed-Loop Digital Micro-Fluxgate Magnetometer”, 13th IEEE International Conference on Electronics, Circuits and Systems, 2006, pages 276-279, an architecture for micro field sensors including a delta-sigma converter with multi-bit feedback DAC for generating a compensation current. The multi-bit feedback is generated based on the digital output of the sensor electronics. This approach suffers from high latency which translates into lower throughput and higher power requirements on the system level.
The three documents cited above provide a discussion on the function of fluxgate sensors and different approaches of signal acquisition. These documents are incorporated herein by reference.